Alzheimer's disease (AD) is a chronic neurodegenerative disease affecting over four million Americans. It is characterized histologically by intracellular neurofibrillary tangles and extracellular aggregates of amyloid peptide containing plaques. In addition, AD brains display robust microglial and astrocyte reactivity. Our long term goal is to identify strategies for limiting microglial activation in the AD brain as a means to prevent inflammatory mechanisms of disease progression. The specific hypothesis of this proposal is that the pre- aggregated, or oligomeric, amyloid peptide stimulates microglial activation in a fashion requiring expression of the parent amyloid precursor protein (APP). This suggests the possibility that APP is a proinflammatory receptor that is activated through formation of its own proteolytic amyloid peptide. We base this hypothesis on the following observations: 1) simulation of ligand binding by antibody cross-linking APP on the surface of microglia or a monocyte cell line stimulates activation of a tyrosine kinase based signaling cascade responsible for increased proinflammatory protein expression;2) stimulation with oligomeric peptide results in a similar profile of tyrosine kinase activation and proinflammatory protein expression;3) oligomer peptide stimulation of microglia and the monocytic cell line is dependent upon expression of APP;and 4) oligomer stimulation of microgliosis and synaptic loss in vivo in mouse brains is also dependent upon expression of APP. Based upon these data, the experimental focus of this application we will define the nature of the APP requirement for oligomer stimulation by determining whether APP is a component of a multireceptor signaling complex needed for oligomeric stimulation of microglia or whether oligomeric Abeta interacts with APP in a direct ligand-receptor fashion to mediate microgliosis. The specific aims will address this hypothesis in vitro in the following manner: 1) Define the role of APP in oligomer-dependent interaction with microglia and monocytes. We will define whether oligomer stimulation results in A) multimerization of APP and/or B) recruitment of APP into a multi- receptor complex. We will also define whether oligomer directly interacts with APP in a classic receptor- ligand fashion. 2) Define the reactive microglial and monocytic phenotype stimulated by oligomeric peptides. We will define the APP dependent signaling response initiated by oligomer stimulation. We will also define the APP dependent secretory profile following oligomer stimulation. Specifically, we will quantitate secretion of specific proinflammatory cytokines and Abeta as well as effects on neuron toxicity.